414 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 16 



1 rem ~ 1 rep for beta particles 



~ 0.25 rep for protons, deuterons, and neutrons 

 ~ 0.1 rep for alpha particles 



Definition: The rem is that dose which, delivered to man (or mammal) 

 exposed to any ionizing radiation, is biologically equivalent to the dose of 

 1 r of x- or gamma radiation (not photoelectrically converted). 



The rem was proposed as a practical unit of dose to circumvent the diffi- 

 culties imposed on other units by the discrepancies in the relative biological 

 effectiveness of various radiations. Hence, the rem is not a measure of 

 energy absorbed or of the ionization produced in tissue; rather, it is a measure 

 of the quantity of radiation that produces certain observable biological 

 effects. The magnitude of the rem in terms of other units is not known with 

 certainty because of the difficulties in evaluating biological effects quanti- 

 tatively. As yet no wholly satisfactory biological indicator is known since 

 different tissues and organs as well as different species of animals exhibit 

 marked variation in radiation resistance. For this reason the relative biologi- 

 cal effectiveness of the same ionization produced, for example, by fast neu- 

 trons and gamma rays has been found experimentally to vary from 2 to 

 about 10 in mammals. An approximate value of 4 for the relative effective- 

 ness of neutrons to gamma rays in this instance was derived from the mean 

 lethal neutron dose for rats [5], assuming that the radiation resistance of 

 rats is not significantly different from man. 



g. n-Unit {proposed by R. S. Strong). A dose of 1 n is approximately 



1 n ~ 2.5 rep 



~ 200 ergs absorbed per gm of tissue 



Definition: A dose of 1 n is delivered to tissue by fast neutrons when the 

 ionization produced in the Victoreen 100 r thimble chamber equals the 

 ionization produced by 1 r of gamma radiation. 



The n-unit provides a convenient means for measuring fast neutron dose 

 with the standard air-wall thimble chamber used in x-ray dosimetry. It 

 does not, however, represent accurately the ionization produced by neutrons 

 in tissue. The dose measured with the thimble chamber represents ionization 

 produced by recoil nuclei in air and from the wall which is airlike in com- 

 position, but in tissue the greater percentage of hydrogen increases the con- 

 version of neutron kinetic energy. On the average protons recoil with a 

 greater fraction of the incident neutron's energy than do oxygen and nitrogen 

 atoms. This effect results in tissue ionization 2 to 2.5 times greater than that 

 measured with the air-wall chamber. 



16.3. Calculation of Beta-particle Dose. The dose delivered to tissue by 

 beta particles (negatrons or positrons) emitted from radioactive isotopes can 



